The present invention relates to a flywheel apparatus using a magnetic bearing and, more particularly, to a flywheel apparatus used for satellite attitude control, i.e., a reaction wheel of a magnetic bearing type.
Flywheel type actuators such as a momentum wheel or a reaction wheel are widely used for controlling the orientation of a satellite. Various types of such flywheel type actuators have already been developed.
The flywheel apparatus generally has a rotor and a motor for driving the rotor at high speed. In a flywheel apparatus of this type, the rotor is rotatably supported through ball bearing.
However, when a flywheel apparatus of the type using ball bearing is used for a flywheel type actuator for controlling satellite attitude, it is disadvantageous that the ball bearing must be lubricated as it wears over time since they are of the contact type. Because, any piece of equipment to be installed on a satellite must be maintenance-free and must have a semi-permanent life. For this reason, it is advantageous to use a magnetic bearing of a noncontact type as the rotor bearing of the flywheel apparatus used for satellite attitude control.
A satellite momentum wheel as described in U.S. Pat. No. 3,955,858 is known as a flywheel apparatus of the type which uses magnetic bearings of a noncontact type. In the magnetic bearings of this conventional satellite momentum wheel, the rotor is radially supported by an uncontrolled type of magnetic circuit or by a passive magnetic circuit utilizing a permanent magnet, and the rotor is axially supported by a controlled type of magnetic circuit or by an active magnetic circuit utilizing an electromagnet.
However, a flywheel apparatus using magnetic bearing of this type still has problems. In the type of magnetic bearing described above, the rotor is radially supported by the passive magnetic circuit. Thus, in order to improve the cross-axial stiffness of the rotor, the stiffness preventing the rotor from rotating around the axis perpendicular to the rotating axis of the rotor, the diameter of the magnetic bearings must be decreased and/or the distance between the rotor and each magnetic bearing along the rotating axis of the rotor must be increased. When the diameter of the magnetic bearings is decreased to improve the cross-axial stiffness of the rotor, the mass of the rotary side of the magnetic bearings cannot be effectively reflected upon the angular momentum of a rotary section (mainly the rotor) of the flywheel apparatus. As a result, the angular momentum of the rotary section of the flywheel apparatus per unit weight becomes, by necessity, small. On the other hand, if the distance between the rotor and the magnetic bearings along the rotating axis of the rotor is increased for the same reason as described above, the size of the apparatus along the rotating axis of the rotor is increased, and the overall flywheel apparatus becomes bulky. Such a bulky flywheel apparatus is not suitable for installation in a satellite.